Liquid crystal display devices are widely used as display devices for various data-processing devices such as computers and televisions. In particular, TFT liquid crystal display devices (hereinafter, also referred to as “TFT-LCDs”) become popular, and expansion of the TFT-LCD market is expected. Such a situation creates a demand for much improved image quality.
Although the present description employs the TFT-LCDs as an example, the present invention may be applicable to passive matrix LCDs, plasma address LCDs, and the like, in addition to the TFT-LCDs. Generally, the present invention is applicable to an LCD which contains a liquid crystal between two substrates each provided with an electrode and which displays an image when a voltage is applied between the electrodes.
The most widely used mode in the TFT-LCDs currently is a mode in which a liquid crystal having positive dielectric anisotropy is horizontally aligned between substrates opposing each other, namely, the TN mode. In a TN liquid crystal display device, the alignment direction of liquid crystal molecules adjacent to one substrate is twisted by 90° to that of liquid crystal molecules adjacent to the other substrate. Such TN liquid crystal display devices are now produced at low cost and have been industrially mature, while they are less likely to achieve a higher contrast ratio. The TN liquid crystal display device may be improved in this respect.
In addition, there are known liquid crystal display devices having another mode in which a liquid crystal having negative dielectric anisotropy is aligned perpendicular to substrates opposing each other, namely the vertical alignment (VA) liquid crystal display devices. In the VA liquid crystal display devices, liquid crystal molecules are aligned almost perpendicular to the surfaces of the substrates when no voltage is applied. Here, the liquid crystal cell hardly shows birefringence and optical rotation, and light passes through the liquid crystal cell while hardly changing in its polarization state. Thus, in the case of the arrangement such that the liquid crystal cell is interposed between two polarizers whose absorption axes are orthogonal to each other, it is possible to display an almost perfectly black screen when no voltage is applied. When a voltage is applied, the liquid crystal molecules are made to be almost parallel to the substrates, the liquid crystal cell shows large birefringence, and the liquid crystal display device displays a white screen. Thus, such a VA liquid crystal display device easily achieves a very high contrast ratio, which is not achieved by the TN liquid crystal display devices.
However, the VA liquid crystal display device is less likely to have a wide viewing angle, and the VA liquid crystal display device may be improved in this respect. This is because as follows.
When no voltage is applied, as mentioned above, the VA liquid crystal display device displays an almost perfectly black screen because the liquid crystal cell hardly shows birefringence and the two polarizers are perfectly orthogonal in the frontal direction (the direction perpendicular to the display surface), but the liquid crystal cell shows birefringence in oblique directions and apparently has phase difference. Further, the two polarizers are apparently not geometrically orthogonal. Thus, light leakage occurs to cause reduction in the contrast ratio, resulting in a narrower viewing angle.
For the above reasons, VA liquid crystal display devices are provided with retardation films in many cases in order to remove excessive retardation of the liquid crystal cell in oblique directions and to maintain orthogonality of the polarizers in a crossed Nicols state in oblique directions. For example, there are disclosed techniques for widening a viewing angle wherein polarizers are disposed on both sides of the perpendicularly aligned liquid crystal cell, and the polarizer and the liquid crystal cell sandwich at least one of the following films: a uniaxial retardation film having an in-plane optic axis and satisfying the relationship of extraordinary index>ordinary index (a positive A plate); a uniaxial retardation film having an out-of-plane (film normal direction) optic axis and satisfying the relationship of extraordinary index<ordinary index (a negative C plate); and a biaxial retardation film (see Patent Documents 1 to 3).
In addition, there are also disclosed the following techniques in which multiple retardation films are used in combination: a technique with combination use of a positive A plate and a positive C plate (see Patent Document 4); a technique with combination use of a negative A plate and a negative C plate (see Patent Document 5); and a technique with combination use of a biaxial retardation plate having birefringence with an in-plane retardation of 250 to 300 nm and an Nz of 0.1 to 0.4 and a biaxial retardation plate having birefringence with an in-plane retardation of 250 to 300 nm and an Nz of 0.6 to 1.1 (see Patent Document 6).
In addition to the VA liquid crystal display devices, there is known another liquid crystal display device wherein an electric field is transversely applied to a homogenious liquid crystal cell in which a liquid crystal is interposed between two substrates each having the surfaces subjected to treatment for homogenious alignment, and thereby liquid crystal molecules are rotated in a plane almost parallel to the substrates to achieve image display; such a device is called as the IPS liquid crystal display device. In the IPS liquid crystal display device, the liquid crystal molecules are always almost parallel to the substrates while the angles formed by the longitudinal directions of the liquid crystal molecules with the absorption axes of the polarizers are changed to display images. Thus, birefringence of the liquid crystal cell is less changed even in oblique directions, and the display device is allowed to have a wide viewing angle.
In the IPS liquid crystal display devices, similar to the case of the VA liquid crystal display devices, two polarizers are disposed orthogonal (in a crossed Nicols state) so as to increase the contrast ratio. Here, the polarizers are apparently not geometrically orthogonal in oblique directions. This causes light leakage upon displaying a black screen, resulting in reduction in the contrast ratio. Thus, the IPS liquid crystal display device may be improved in this respect.
In order to prevent such reduction in the contrast ratio, the IPS liquid crystal display device is also provided with a retardation film. For example, there is disclosed a technique in which the polarizer and the liquid crystal cell sandwich an appropriate biaxial retardation film having an adjusted in-plane retardation and thickness-direction retardation (in-plane retardation is 190 to 390 nm, Nz=0.3 to 0.65) (see Patent Document 7).
There is also disclosed a technique in which multiple retardation films are used in combination, such as a technique in which a negative uniaxial A plate (optic axis 0°) and a positive uniaxial A plate (optic axis 90°) are placed between the viewing-side polarizer (absorption axis 90°) and the back-side polarizer (absorption axis 0°) (see Non-Patent Document 1).
Further, there is disclosed a multilayer polarizing film comprising a positive almost uniaxial optical film, a negative optical film, and a polarizing film stacked in this order, wherein the slow axes thereof are almost parallel to the absorption axes thereof and the retardation of at least one of the almost uniaxial optical film and the negative optical film is made to show reverse wavelength dispersibility (see Patent Document 8).
In addition, there is disclosed a multilayer polarizing film comprising a negative almost uniaxial optical film, a positive optical film, and a polarizing film stacked in this order, wherein the slow axes thereof are almost orthogonal to the absorption axes thereof and the retardation of at least one of the negative almost uniaxial optical film and the positive optical film is made to show reverse wavelength dispersibility (see Patent Document 9).
[Patent Document 1]
U.S. Pat. No. 6,141,075
[Patent Document 2]
U.S. Pat. No. 6,661,486
[Patent Document 3]
U.S. Pat. No. 7,057,689
[Patent Document 4]
WO 06/001448
[Patent Document 5]
Japanese Kohyo Publication 2006-514754
[Patent Document 6]
Japanese Kokai Publication 2001-350022
[Patent Document 7]
Japanese Kokai Publication H11-305217
[Patent Document 8]
Japanese Kokai Publication 2007-232873
[Patent Document 9]
Japanese Kokai Publication 2007-232874
[Non-Patent Document 1]
XiNzhu, et al., “Super Wide View In-plane Switching LCD with Positive and Negative Uniaxial A-Films Compensation”, SID 05 DIGEST, p. 1164-1167